Objects appear in the viewport and preview render, but not in the outliner or final render
As mentioned in the comments, you most likely have the eyes set as not visible in the render. This is shown in the outliner with the camera to the right of the object, the eye is for viewport visibility, which also effects viewport preview rendering while the camera only affects the final render.

To find an item in the outliner you have a few options, if they are named well you can search for them -



If you haven't named things, you can select it in the viewport and the item will be highlighted in the outliner, with a large list of items press . with the mouse over the outliner to zoom to the item, this will expand any parent objects to show the item.



Once you locate the object in the outliner, click the camera to make it visible in the render.